Spark-ignited internal combustion engine liquid fuel, i.e. so-called gasoline, is generally manufactured by blending together petroleum fractions obtained by distilling crude oil, or gasoline bases obtained by processing such petroleum fractions. Hydrocarbons contained in a gasoline generally have approximately 4 to 10 carbon atoms.
Octane number is a typical gasoline performance index. Out of bases having as a principal component thereof hydrocarbons having 7 or more carbon atoms, modified gasoline bases (reformates) having as a principal component thereof aromatic hydrocarbons having 7 to 9 carbon atoms such as toluene are used as gasoline bases exhibiting a high octane number.
For a gasoline base that has as a principal component thereof hydrocarbons having 5 and 6 carbon atoms and exhibits a high octane number, a manufacturing method involving an isomerization process is known. This is a method in which the octane number is increased by isomerizing straight-chain saturated hydrocarbons into branched saturated hydrocarbons, this being because straight-chain saturated hydrocarbons exhibit a low octane number, whereas branched saturated hydrocarbons having the same number of carbon atoms exhibit a high octane number. As catalysts used in such an isomerization process, (1) solid acid catalysts in which sulfuric groups are supported on zirconia, and (2) solid acid catalysts that use a zeolite, are known.